Channels of the Transient Receptor Superfamily (TRP), such as TRPV1, TRPM8 and TRPA1, are non-selective cation channels that conduct calcium and sodium into a range of cell types in mammals. They are present on sensory neurons, and were initially identified as having a role in nociception because of their responsiveness at the molecular level to plant secondary metabolites that are nociomimetic (e.g., capsaicin) and to compounds that are otherwise pungent and mimic burning or cooling sensations (e.g., allicin, cinnamaldehyde, menthol).
Because of their role in nociception, TRP channels have been identified as targets for treating pain disorders. Both antagonism and agonism of the TRP channel have been exploited for pain management. For example, TRPV1 antagonists have utility in acute analgesia. For chronic pain management, TRPV1 agonists are typically used. This latter strategy exploits the fact that continued TRPV1 receptor agonism causes desensitization at the cell surface (receptor internalization, degradation and recycling). Prolonged agonism of TRPV1 also leads to calcium and sodium cationic overload of the TRPV1-containing sensory neuron, leading to cell death.
In practice, the use of TRPV1 agonists to effect desensitization involves topical application of high levels of a well-known TRPV1 agonist, capsaicin, repeatedly over time to the affected area. This therapeutic approach has the benefit of efficacy and low cost. However, it also has weaknesses.
First, high affinity and high specificity TRPV1 agonists target only TRPV1-containing nociceptors, leaving other sensory neurons and TRP channels involved in pain untouched. Second, use of high affinity and high specificity TRPV1 agonists such as capsaicin causes high levels of discomfort during initial treatment, in the period prior to desensitization. It is for this reason that post-herpetic pain is currently not addressable using TRPV1-mediated desensitization due to the highly irritant nature of the therapy on sensitive areas such as the gastric mucosa and reproductive tract mucosa. Third, capsaicin-mediated desensitization treatments are limited to topical use; visceral pain, headache and certain musculoskeletal pain disorders are not addressed by this therapy.
There is, therefore, a need for therapeutic TRPV1 ligands, such as TRPV1 agonists, that are lower affinity than capsaicin. Such lower affinity ligands should cause reduced pain during desensitization, thereby allowing topical treatment of sensitive body areas. There is a need for TRPV1 ligands with broader target specificity, able to target multiple types of TRP-bearing nociceptors, thereby improving the degree of tissue desensitization. There is also a need for TRPV1 ligands suitable for systemic administration in addition to topical application.
Such new medications would also be useful for the treatment of various diseases associated with TRPV1 other than pain. While TRP channels were first shown to be involved in pain and nociception, they now known to have various other physiological roles, suggesting that they can be a target for treatment of other diseases. For example, TRP channels have been identified as a target for treatment of cardiovascular disease; targeted pharmacological inhibition of TRPV1 has been shown to significantly diminish cardiac hypertrophy in a mouse model. See U.S. Pat. No. 9,084,786. Chronic downregulation of TRPV1 levels by receptor desensitization with a TRPV1 agonist would therefore be expected to similarly protect, and potentially rescue, cardiac hypertrophy and its associated symptoms and outcomes (cardiac remodeling, cardiac fibrosis, apoptosis, hypertension, or heart failure). However, there is currently no TRPV1 agonist suitable for systemic administration and suitable for chronic downregulation of TRPV1 in a visceral organ, and there is therefore a need to develop such approaches in an analogous manner to the chronic pain approaches described above.
Thus, there exists a need to find new compounds that act as TRPV1 antagonists and TRPV1 agonists. Such new compounds would provide novel and more effective ways of treating various diseases associated with the TRPV1 channel, including pain disorders and cardiovascular diseases.